1. Technical Field
This disclosure generally relates to opto-electrical devices such as organic light-emitting diodes (OLEDs) and photovoltaic (PV) cells.
2. Description of the Related Art
An OLED emits light in response to an electric current. The light generation mechanism is based on radiative recombination of excitons of electrically excited organic compound(s). FIG. 1 shows a conventional OLED (10) formed on a substrate (12). An anode (14) is disposed on the substrate. The light-emitting layer takes the form of an organic stack (16) that includes a thin film of electroluminescent chemical compounds (18) flanked by two charge injection layers (20 and 22, one for electron injection and one for hole injection). A cathode (24) is disposed on the organic stack (16).
The cathode and anode provide the contacts for an external circuitry to supply an electrical current, which in turn generates light (26) in the organic stack (16). In general, in a bottom-emitting OLED, the anode (14) and the substrate (12) shall be transparent, allowing the internally generated light (26) to exit from the substrate (12). In a top-emitting OLED, e.g., an OLED display, the cathode (24) shall be transparent, allowing the internally generated light (26) to exit from the cathode. The OLED may be entirely transparent, whereby both the cathode and the anode are transparent. A conventional transparent electrode is indium tin oxide (ITO).
The internally generated light (26) propagates via various modes. Using a bottom-emitting OLED as an example, FIG. 2 schematically shows certain paths of the light propagation. It is desired that the light propagates via an external mode (30), i.e., by traveling through the transparent anode (14) and exiting from the substrate (12). However, not all the light generated from the organic stack is capable of existing via the external mode. Instead, depending on the light incident angle on a given interface, a substantial portion of the light may propagate via a number of waveguide modes. More specifically, at an interface where light enters from a medium of higher refractive index to one of lesser refractive index, a total internal reflection occurs at the interface of the two media for certain light incident angles, whereby no light exits through the interface. As a result, a substantial amount of the light generated from the organic stack is waveguided. One waveguide mode is a combined mode of the ITO layer (14) and the organic stack (16), both of which have comparably high refractive indices. As shown in FIG. 2, light (32) is totally reflected at the interface (34) between the substrate (12) and the combined ITO-organic stack. Another mode is a substrate mode, where the light (38, 40) is reflected at the interface (36) of the substrate and air. As shown, the waveguided light (38) may also exit from the edges of an OLED.
Thus, a critical parameter of OLED performance is the external coupling (“out-coupling”) efficiency, which is the ratio of photons emitted externally over photons generated. There is a need for improving the light out-coupling of OLEDs.